An outbreak of nosebleeds among pupils in two Chinese provinces at the start of the new school year is being investigated to see whether toxic chemicals in running tracks are to blame.

The incidents happened just as the Ministry of Education began rolling out a set of upgraded standards, released in May for implementation in November, for synthetic materials used in the sports grounds of infant and primary schools across the nation.

The regulations were developed in response to widespread health concerns about toxic plastic in running tracks after several high-profile cases in 2016 when pupils experienced nosebleeds, headaches and dizziness after spending time near the tracks.

The latest cases occurred in the city of Handan, northern China’s Hebei province, and in Wuhan, capital of central Hubei province. Several first-grade pupils at a junior school in Handan began experiencing nosebleeds earlier this month, according to the Yanzhao Evening News.

They were all in first-floor classrooms next to a running track that had been retrofitted with a plastic coating in the weeks leading up to the start of school, the report said.

Meanwhile in Wuhan, officials are investigating the outdoor track and basketball courts at a primary school after a number of pupils there experienced nosebleeds, according to Hubei Daily.

Parents noticed a strange smell coming from the track and sports area, and demanded an investigation, the report said.

In both cases, the areas have been cordoned off and the plastic surfaces covered while the local governments await toxicology results.

The new rules for synthetic materials used in school sports grounds have been in the works since a spate of health incidents from 2014 to 2016.

Students from at least 32 schools across 12 provinces experienced symptoms suspected to have been caused by toxic plastic in the schools’ running tracks, according to a state media tally.

Lax regulations have meant that materials in school tracks could be compliant, but still dangerous for children, according to Wei Wenfeng, founder of the Hangzhou-based child safety-focused consumer protection firm DaddyLab, who began investigating running track toxicity in 2015.

“The Ministry of Education became aware of this in 2016, and began to upgrade the standards for running track systems,” he said, referring to the regulations released this year.

Wei said the new regulations, which ban a wider list of chemicals, were a crucial step forward in protecting children. But he remained concerned that factories might find new and potentially harmful chemicals to replace those on the restricted list, perpetuating the problem.

“The standard should not only be to publish a blacklist, but to give manufacturers a ‘white list’, to let them know that they can add only these specific chemicals to produce and manufacture running tracks for schools,” he said.

“If chemicals are not on the list, then they can’t be put into the product.”

In the most prominent case from 2016, Beijing No.2 Experimental School decided to rip up its entire playground after health checks found 137 pupils had dangerous levels of benzene and formaldehyde in their blood, which parents and educators believed to be from toxic odours emitted from a new school track.

That case prompted an investigation by state broadcaster CCTV, which traced the materials used for some Beijing running tracks back to dozens of unlicensed mills in Hebei province, where discarded car tyres, old cables and other industrial waste were being processed into track materials and sold to contractors.

Immediately after that report, the education ministry issued a statement outlining an action plan that included guidelines for school inspections and a stated intention to ramp up the supervision of the manufacturing supply chain and oversight of schools’ construction contracts.

The new regulations are the most comprehensive action so far to combat dangerous manufacturing standards.